1. Field of the Invention
The present invention relates to a method for producing a higher silane useful as a starting material for producing semiconductor silicon and the like. Since higher silanes such as trisilane, tetrasilane, pentasilane, hexasilane and heptasilane are more easily decomposed than lower silanes such as monosilane and are low in loss by evaporation during film formation, when they are used as starting materials for producing polycrystalline silicon, epitaxial silicon or amorphous silicon, it is possible to attain a decrease in the film forming temperature, an improvement in the film forming rate and an increase in the formed film yield. Therefore, it is expected that they will be widely used.
2. Description of the Related Art
Many methods for producing higher silanes are known as shown below: (1) Reduction of silicon by hydrogen, using hydrogen sulfide or a metal sulfide as a catalyst (Japanese Examined Patent Publication (Kokoku) No. 36-21507, etc.), (2) Reduction of a silicon compound having a plurality of chlorines (Japanese Unexamined Patent Publication (Kokai) No. 57-27915, etc.), (3) Reaction between an oxygen compound of silicon having Si-H or Si-Si bond and a hydride, an alkoxide or an amalgam of an alkali (earth) metal (Japanese unexamined Patent Publication (Kokai) Nos. 60-255612, 60-251114, 60-260418, etc.), (4) Reaction between a magnesium-silicon alloy and an acid (Japanese Unexamined Patent Publication (Kokai) No. 60-141613, etc.), (5) Electric discharge in monosilane gas (Japanese Unexamined Patent Publication (Kokai) Nos. 62-132720 and 62-132721, etc.), (6) Dehydrogenation condensation of hydrosilanes using a platinum-group metal complex as a catalyst (Japanese Unexamined Patent Publication (Kokai) Nos. 1-198631 and 2-184513), (7) Dehydrogenation condensation of hydrosilanes using a lanthanoid complex as a catalyst (Japanese Unexamined Patent Publication (Kokai) No. 5-32785), and (8) Pyrolysis of tri-, tetra- and penta-silanes using glass wool or palladium/silica and glass wool as a catalyst (German Patent No. 2139155). The present inventors disclosed a low cost and safe method for producing higher silanes by pyrolizing monosilane with an alumina catalyst or without a catalyst (Japanese Unexamined Patent Publication (Kokai) Nos. 3-183613 and 3-183614).
However, the above known methods have many problems to be solved before commercial application is possible.
That is, the method by reducing silicon with hydrogen using hydrogen sulfide or a metal sulfide as a catalyst cannot exclude contamination by hydrogen sulfide in the product.
In the method using reducing a silicon chloride compound, the starting material of a higher chlorinated silane compound is expensive because of the difficulty in manufacture, and when an alkali metal is used as a reducing agent, control of the reaction is difficult since the reaction is severe, and an alkali metal, having a high possibility of burning and therefore being difficult to handle, must be used in a large amount. On the other hand, in the method by reducing alkyl aluminum halide/trialkylaluminum, an organic silicon compound, which is difficult to remove, contaminates the product.
In the method using a reaction between an oxygen compound of silicon having Si-H or Si-Si bond and a hydride, an alkoxide or an amalgam of an alkali (earth) metal, the alkali (earth) metal or its hydride, etc., having a high possibility of burning and being difficult to handle, must be used in a large amount and, in particular, in the amalgam reduction method, commercial application involves problems since poisonous mercury must be used.
In the method using a reaction between a magnesium-silicon alloy, the yield is low since oxygen-containing impurities such as siloxane are formed and the yield of the higher silane is lower since monosilane is formed in a large amount.
In the method using electric discharge in a monosilande gas, there are many problems, e.g., the apparatus is expensive and requires much energy, the conversion rate is low, and the selectivity of the higher silane among the decomposed compounds is low with the remainder being a powder of hydrogen-containing silicon.
In the method using dehydrogenation condensation of hydrosilanes using a platinum-group metal complex as a catalyst, the catalyst activity is low, the by-product amount is high, the products are only up to trisilane, and the production of the higher silane is low.
In the method disclosed by the present inventors, an increase in the yield of disilane and trisilane is attained by preventing production of tetra- or higher-silanes.
In the method using dehydrogenation condensation of hydrosilanes using a lathanoid complex as a catalyst, the reaction takes a few days and the productivity is low.
In the method using pyrolysis of tri-, tetra- and penta-silanes using glass wool or palladium/silica and glass wool as a catalyst, a higher silane is used to obtain a further higher silane and the starting material is expensive. Therefore, this method cannot be commercially used at present.
The object of the present invention is to provide a method for producing a liquid higher silane comprising a large amount of components in a range of from trisilane to nonasilane by pyrolizing a lower silane comprised mainly of easily available monosilane or disilane.